Exclusive photoproduction of $pi^0$ up to large values of Mandelstam variables $s, t$ and $u$ with CLAS

Exclusive photoproduction cross sections have been measured for the process $gamma p rightarrow ppi^0(e^+e^-(gamma))$ with the Dalitz decay final state using tagged photon energies in the range of $E_{gamma} = 1.275-5.425$ GeV. The complete angular distribution of the final state $pi^0$, for the entire photon energy range up to large values of $t$ and $u$, has been measured for the first time. The data obtained show that the cross section $dsigma/dt$, at mid to large angles, decreases with energy as $s^{-6.89pm 0.26} $. This is in agreement with the perturbative QCD quark counting rule prediction of $s^{-7} $. Paradoxically, the size of angular distribution of measured cross sections is greatly underestimated by the QCD based Generalized Parton Distribution mechanism at highest available invariant energy $s=11$ GeV$^2$. At the same time, the Regge exchange based models for $pi^0$ photoproduction are more consistent with experimental data.